Wavefront measuring apparatus and wavefront measuring method

ABSTRACT

A wavefront measuring apparatus and a wavefront measuring method are capable of performing optical measurement on a test optical system, including an immersion optical system, with comparable ease of handling to that of the conventional measuring method using a concave member, and substantially independently of reflection that may occur at the surface closest to the test optical system among the surfaces of an optical member for reflecting light exiting from the test optical system. The wavefront measuring apparatus has a light source, a reference light path in which a reference member for producing reference light is disposed, and a test light path in which the test optical system is disposed. A plano-convex optical member with a wall thickness approximately equal to the radius of curvature of a convex surface thereof is disposed in the test light path in such a manner that a plane surface thereof faces toward the test optical system. The space between the test optical system and the plano-convex optical member is filled with a liquid.

This application claims benefit of Japanese Application No. 2001-51408filed in Japan on Feb. 27, 2001, the contents of which are incorporatedby this reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a wavefront measuring apparatus andmethod for evaluating the optical performance of an optical system(hereinafter referred to as “test optical system”) having a lens, aprism or other optical element. More particularly, the present inventionrelates to a wavefront measuring apparatus and method capable ofconducting an optical performance evaluation without being affected byunwanted reflected light or irrelevant aberrations even when the testoptical system is an immersion optical system.

2. Discussion of Related Art

A conventional measuring apparatus used to evaluate the performance of atest optical system is shown in FIG. 8. The measuring apparatus has aFizeau interferometric optical system, which comprises a light source 1,a light-collecting lens 2, a pinhole 3, a beam splitter 4, a collimatorlens 5, a semitransparent mirror 6 as a reference member, a concavemember 8, a spatial filter 9, a relay lens 10, and a CCD camera 11 as animage pickup device. An objective 7 as a test optical system is placedbetween the semitransparent mirror 6 and the concave member 8.

Light emitted from the light source 1 is once collected on the pinhole 3through the light-collecting lens 2. Light passing through the apertureof the pinhole 3 is formed into a parallel beam through the collimatorlens 5 and incident on the semitransparent mirror 6. The semitransparentmirror 6 produces reflected light and transmitted light according to apredetermined reflectance (or transmittance). Of the light incident onthe semitransparent mirror 6, light reflected from the semitransparentmirror 6 is herein referred to as “reference light”. The reference lightpasses through the collimator lens 5 and is then reflected by the beamsplitter 4. Then, the reference light passes successively through thespatial filter 9 and the relay lens 10 to enter the CCD camera 11.

Meanwhile, light passing through the semitransparent mirror 6 enters theobjective 7. Herein, light passing through the test optical system(objective 7) is referred to as “test light”. If the objective 7 hasaberrations, the wavefront of the test light is deformed. After beingcollected through the objective 7, the test light diverges as it isincident on the concave member 8. The concave surface of the concavemember 8 has such a curvature that the direction of reflection of theincident light is coincident with the direction of incidence of thelight. Accordingly, the test light reflected from the concave surface ofthe concave member 8 reenters the objective 7 and exits therefrom in theform of a parallel beam. The parallel beam exiting from the objective 7passes through the semitransparent mirror 6 and enters the CCD camera 11in the same way as the reference light.

Light reflected from the beam splitter 4 includes the reference lightand the test light. Therefore, interference occurs between the referencelight and the test light, and interference fringes are formed on the CCDcamera 11 by the relay lens 10. Thus, the state of the interference canbe observed. It should be noted that interference fringes suitable formeasurement can be obtained by moving the semitransparent mirror 6 alongthe optical axis. The interference fringes formed on the CCD camera 11contain information concerning aberrations of the objective 7.Therefore, it is possible to obtain aberrations of the objective 7, e.g.wavefront aberrations, by analyzing the interference fringes.

It should be noted that, in the arrangement shown in FIG. 8, the opticalpath through which the reference light passes, i.e. from thesemitransparent mirror 6 to the beam splitter 4 (or the CCD camera 11),corresponds to the reference light path. The optical path through whichthe test light passes, i.e. from the concave member 8 to the beamsplitter 4 (or the CCD camera 11), corresponds to the test light path.The optical path from the light source 1 to the beam splitter 4 is acommon light path. The optical path from the beam splitter 4 to the CCDcamera 11 may also be said to be a common light path.

A similar technique of measuring the optical characteristics of a testoptical system by utilizing an interferometric optical system as in thecase of FIG. 8 is disclosed in Japanese Patent Application UnexaminedPublication Number [hereinafter referred to as “JP(A)”] Hei 10-90113. InJP(A) Hei 10-90113, a hemispherical lens is used in place of the concavemember 8. The reason for using the hemispherical lens is that thedivergence angle of the beam can be reduced according to the refractiveindex of the vitreous material of the hemispherical lens. Thus, theoptical member (the concave member 8 in FIG. 8 or the hemispherical lensin JP(A) Hei 10-90113) for reflecting the light exiting from the testlens back to it can be produced in a compact and lightweight structure.Further, it is possible to dispense with a compensating plate by takinginto consideration the cover glass thickness when setting the thicknessof the hemispherical lens.

JP(A) Hei 9-184787 discloses a technique that allows measurement of atest lens even when it is an immersion objective. In JP(A) Hei 9-184787,three test lenses are prepared, and wavefront measurement is carried outfor each pair of the three lenses in such a manner that the two lensesare placed to face each other. In this way, a total of threecombinations of the lenses are subjected to wavefront measurement, and awavefront is determined by computation. In this case, if coordinatesystems used for the measurement of the three combinations of the lensesare not held in a predetermined positional relationship, accuratecomputation cannot be executed. Therefore, the disclosed techniquecontrives that the coordinate systems should be held in a predeterminedpositional relationship. JP(A) Hei 9-184787 discloses that when the testlenses are immersion objectives, the space between the objectives placedto face each other is filled with a liquid.

JP(A) Hei 10-90113 certainly allows the optical element itself to beproduced in a compact structure when a hemispherical lens is used, butit has the problem that the influence of reflected light from the planeportion of the hemispherical lens cannot be avoided. For example, asshown in FIG. 1 of JP(A) Hei 10-90113, air is present between theforemost lens element in the test lens and the hemispherical lens.Therefore, about 4% of light exiting from the test lens and entering thehemispherical lens is reflected at the plane portion of thehemispherical lens. The reflectance at the convex surface of thehemispherical lens is also about 4%.

Accordingly, light reflected from the plane surface of the hemisphericallens, which is not originally necessary for the measurement, is added tothe test light reflected from the convex surface of the hemisphericallens. If the reflectance of a plane plate is set at about 4%, thereference light and the test light become approximately equal inintensity to each other. Therefore, interference fringes formed byinterference are substantially the same as those which should originallybe obtained, and thus have a fairly high contrast. However, if reflectedlight occurs at the plane portion of the hemispherical lens, thisreflected light also interferes with the reference light, together withthe test light. At this time, the intensity of the reflected light isapproximately equal to that of the test light. Accordingly, the contrastof interference fringes produced by the reflected light is approximatelyequal to the contrast of interference fringes produced by the referencelight and the test light.

Thus, with the arrangement disclosed in JP(A) Hei 10-90113, not onlyinterference fringes produced by the reference light and the test lightbut also interference fringes produced by the reference light and thelight reflected from the plane portion of the hemispherical lens areformed on the CCD camera. These two interference fringe patterns areobtained in the form of coherent summation (summation of amplitudes).Therefore, the two interference fringe patterns cannot be separated fromeach other after they have been imaged. In other words, it is impossibleto remove the interference fringes produced by the reference light andlight reflected from the plane portion of the hemispherical lens fromthe interference fringes produced by the reference light and the testlight, which are originally necessary for the measurement. Consequently,accurate wavefront measurement cannot be carried out.

It is conceivable to provide the plane portion with an antireflectioncoating or the like for the purpose of reducing reflection at the planeportion. However, when the numerical aperture of the test optical systemis high, the angle of light incident on the plane surface is about 70°at the maximum. It is very difficult in general practice to provide anantireflection coating capable of making the reflectance nearly zerowith respect to a wide range of incident angles, i.e. from 0° to 70°.Further, because air is present between the test lens and thehemispherical lens, if the hemispherical lens deviates from a perfecthemisphere (exclusive of the amount of compensation made by the coverglass), the deviation appears as aberration, making it impossible toperform satisfactory measurement.

On the other hand, the technique disclosed in JP(A) Hei 9-184787 takesinto consideration not only a dry optical system but also an immersionoptical system as test optical systems but needs at least three sets ofoptical systems as test optical systems. Further, it is necessary toadjust the coordinate system for each combination of test opticalsystems. Accordingly, the optical system requires very severeadjustment. Thus, it is difficult to adjust the optical system.

SUMMARY OF THE INVENTION

The present invention was made in view of the above-described problems.

Accordingly, an object of the present invention is to provide awavefront measuring apparatus and a wavefront measuring method that arecapable of performing wavefront measurement on a test optical systemwith comparable ease of handling to that of the conventional measuringmethod using a concave member.

Another object of the present invention is to provide a wavefrontmeasuring apparatus and a wavefront measuring method that are capable ofperforming favorable measurement on test optical systems even in thecase of an immersion optical system in which the space between theobject and the optical system is filled with a liquid.

Still another object of the present invention is to provide a wavefrontmeasuring apparatus and a wavefront measuring method that are capable ofperforming favorable measurement substantially independently ofreflection that may occur at the surface closest to a test opticalsystem among the surfaces of an optical member for reflecting lightexiting from the test optical system.

To solve the above-described problems, the present invention provides awavefront measuring apparatus for measuring the wavefront of lightpassing through a test optical system. The wavefront measuring apparatushas a light source and a reference light path in which a referencemember for producing reference light is disposed. The wavefrontmeasuring apparatus further has a test light path in which the testoptical system is disposed. A plano-convex optical member is disposed inthe test light path in such a manner that a plane surface thereof facestoward the test optical system. The plano-convex optical member has awall thickness approximately equal to the radius of curvature of aconvex surface thereof. The space between the test optical system andthe plano-convex optical member is filled with a liquid.

In addition, the present invention provides a wavefront measuringapparatus for measuring the wavefront of light passing through a testoptical system. The wavefront measuring apparatus has a light source anda reference light path in which a reference member for producingreference light is disposed. The wavefront measuring apparatus furtherhas a test light path in which the test optical system is disposed. Aplano-convex optical member is disposed in the test light path in such amanner that a plane surface thereof faces toward the test opticalsystem. The plano-convex optical member has a wall thicknessapproximately equal to the radius of curvature of a convex surfacethereof. The optical path length of the test light path and that of thereference light path are approximately equal to each other. When thecoherence length of the light source is denoted by L and the wallthickness of the plano-convex optical member is denoted by d and furtherthe refractive index of the plano-convex optical member is denoted byn_(opt), the following condition (2) is satisfied:

L<2×n_(opt)×d  (2)

In addition, the present invention provides a wavefront measuring methodfor measuring the wavefront of light passing through a test opticalsystem by using a measuring optical system having a light source and areference light path in which a reference member for producing referencelight is disposed. The measuring optical system further has a test lightpath in which the test optical system is disposed. A plano-convexoptical member is disposed in the test light path in such a manner thata plane surface thereof faces toward the test optical system. Theplano-convex optical member has a wall thickness approximately equal tothe radius of curvature of a convex surface thereof. The space betweenthe test optical system and the plano-convex optical member is filledwith a liquid.

In addition, the present invention provides a wavefront measuring methodfor measuring the wavefront of light passing through a test opticalsystem by using a measuring optical system having a light source and areference light path in which a reference member for producing referencelight is disposed. The measuring optical system further has a test lightpath in which the test optical system is disposed. A plano-convexoptical member is disposed in the test light path in such a manner thata plane surface thereof faces toward the test optical system. Theplano-convex optical member has a wall thickness approximately equal tothe radius of curvature of a convex surface thereof. The optical pathlength of the test light path and that of the reference light path areapproximately equal to each other. When the coherence length of thelight source is denoted by L and the wall thickness of the plano-convexoptical member is denoted by d and further the refractive index of theplano-convex optical member is denoted by n_(opt), the followingcondition (2) is satisfied:

L<2×n_(opt)×d  (2)

Still other objects and advantages of the invention will in part beobvious and will in part be apparent from the specification.

The invention accordingly comprises the features of construction,combinations of elements, and arrangement of parts which will beexemplified in the construction hereinafter set forth, and the scope ofthe invention will be indicated in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a first embodiment of the wavefrontmeasuring apparatus according to the present invention.

FIG. 2 is a diagram showing a second embodiment of the wavefrontmeasuring apparatus according to the present invention.

FIG. 3 is a diagram showing a third embodiment of the wavefrontmeasuring apparatus according to the present invention.

FIG. 4 is a diagram showing a fourth embodiment of the wavefrontmeasuring apparatus according to the present invention.

FIG. 5 is a diagram showing a fifth embodiment of the wavefrontmeasuring apparatus according to the present invention.

FIG. 6 is a diagram showing a sixth embodiment of the wavefrontmeasuring apparatus according to the present invention.

FIG. 7 is a diagram showing a seventh embodiment of the wavefrontmeasuring apparatus according to the present invention.

FIG. 8 is a diagram showing a conventional wavefront measuringapparatus.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Wavefront measuring apparatus according to embodiments of the presentinvention measure the wavefront of light passing through a test opticalsystem by utilizing interference. In a first arrangement according tothe embodiments of the present invention, the wavefront measuringapparatus has, in order to produce interference, a light source, areference light path in which a reference member for producing referencelight is disposed, and a test light path in which the test opticalsystem is disposed. A plano-convex optical member is disposed in thetest light path in such a manner that a plane surface thereof facestoward the test optical system. The plano-convex optical member has awall thickness approximately equal to the radius of curvature of aconvex surface thereof. The space between the test optical system andthe plano-convex optical member is filled with a liquid.

In the first arrangement, the space between the test optical system andthe plano-convex optical member (hereinafter referred to as“plano-convex member”) is filled with a liquid to reduce the differencebetween the refractive index at a side of the plane surface of theplano-convex member closer to the test optical system and the refractiveindex at the inside of the plano-convex member. The refractive indexdifference varies according to the refractive index of the plano-convexmember and the refractive index of the liquid. As compared to anarrangement in which air is present between the test optical system andthe plano-convex member, the first arrangement is much superior incapability of reducing the reflection of light at the plane surface ofthe plano-convex member, provided that the plano-convex members in thetwo arrangements have the same refractive index.

Thus, the first arrangement can minimize the generation of reflectedlight at the plane surface of the plano-convex member (the reflectedlight will hereinafter be referred to as “noise light”). If thegeneration of noise light is minimized, substantially no interferencefringes are produced by the noise light and the reference light.Accordingly, the first arrangement makes it possible to detect onlyinterference fringes produced by the test light and the reference light,which are originally necessary for the measurement.

Further, minimization of the refractive index difference between theliquid and the plano-convex member results in a marked reduction in theamount of aberration produced by the plano-convex member even if it isnot a perfect hemisphere. Let us, for example, assume that the centerwall thickness of the plano-convex member is larger than that of acorresponding perfect hemisphere. This is equivalent to a situation inwhich a plane-parallel plate is present between the test optical systemand the plano-convex member. If the refractive index difference betweenthe liquid and the plano-convex member is large, aberration occurs at aportion of the plano-convex member that corresponds to a plane-parallelplate. However, if the refractive index difference between the liquidand the plano-convex member is sufficiently small, the portioncorresponding to a plane-parallel plate can be regarded as beingapproximately equivalent to the liquid. If there is no refractive indexdifference, the arrangement is equivalent to a situation in which noplane-parallel plate is present. Even when there is a refractive indexdifference, if it is small, the thickness that is converted to aplane-parallel plate is correspondingly small. Therefore, the amount ofaberration produced by the plano-convex member is very small. It shouldbe noted that the above discussion also holds true for a case where thecenter wall thickness of the plano-convex member is smaller than that ofa corresponding perfect hemisphere.

Thus, by minimizing the refractive index difference between the liquidand the plano-convex member, it becomes unnecessary to form theplano-convex member into a perfect hemisphere and hence possible torelax the requirement for the manufacturing accuracy when theplano-convex member is produced. Consequently, the productivity of theplano-convex member can be improved. In addition, the first arrangementis superior in that it is unnecessary to prepare a plurality of testoptical systems, and that wavefront measurement can be performed on animmersion optical system with substantially the same operability as inthe measurement with an optical system using a concave member.

It should be noted that the liquid used in the first arrangement may beimmersion oil such as that used in microscopes, for example. However,when the test optical system is an immersion optical system, it hasgenerally been designed on the assumption that the optical system willbe used through a predetermined immersion liquid. Therefore, the liquidused in the first arrangement should preferably be the same immersionliquid that is assumed to be used by the test optical system.Alternatively, it is preferable to use a liquid having approximately thesame refractive index as that of the immersion liquid.

In the first arrangement, it is preferable to satisfy the followingcondition (1):

|n_(liq)−n_(opt)|≦0.1  (1)

where n_(liq) denotes the refractive index of the liquid, and n_(opt)denotes the refractive index of the plano-convex optical member.

The condition (1) is a condition for further minimizing the generationof noise light. Let us explain the condition (1). Measurement of ahigh-precision optical system (i.e. an optical system satisfactorilycorrected for aberrations) needs to minimize error components occurringin the apparatus. There are various error components. Among them, noiselight is a matter of our concern. The condition (1) is necessary tosatisfy in order to further minimize the generation of noise light. If|n_(liq)−n_(opt)| exceeds the upper limit, i.e. 0.1, the intensity ofnoise light becomes excessively high. Consequently, unwantedinterference fringes (noise component) produced by the noise light areformed in addition to interference fringes that are originallynecessary. Therefore, it is unfavorable to set |n_(liq)−n_(opt)| inexcess of 0.1.

In contrast, if a liquid and a plano-convex member are selected so thatthe condition (1) is satisfied, the intensity of noise light can bereduced sufficiently. As a result, it becomes possible to minimize thegeneration of unwanted interference fringes and hence possible to obtainonly interference fringes originally required. Further, even if theplano-convex member is not a perfect hemisphere, it is possible tominimize the amount of aberration produced by the plano-convex member.Accordingly, the first arrangement is particularly useful formeasurement of a high-precision optical system.

In a second arrangement according to the embodiments of the presentinvention, the wavefront measuring apparatus has a light source, areference light path in which a reference member for producing referencelight is disposed, and a test light path in which a test optical systemis disposed. A plano-convex optical member is disposed in the test lightpath in such a manner that a plane surface thereof faces toward the testoptical system. The plano-convex optical member has a wall thicknessapproximately equal to the radius of curvature of a convex surfacethereof. The optical path length of the test light path and that of thereference light path are set approximately equal to each other. When thecoherence length of the light source is denoted by L and the wallthickness of the plano-convex optical member is denoted by d and furtherthe refractive index of the plano-convex optical member is denoted byn_(opt), the following condition (2) is satisfied:

L<2×n_(opt)×d  (2)

With the second arrangement, even if noise light is generated, formationof interference fringes by the noise light is prevented. The condition(2) is a condition for preventing formation of interference fringes bynoise light.

The first arrangement suppresses reflection at the plane surface of theplano-convex member by filling the space between the test optical systemand the plano-convex member with a liquid. However, there are caseswhere it is very difficult to suppress the reflection from the viewpointof principle. For example, when the plano-convex member is made of glassand the test optical system is a water-immersion optical system, thereflection is very difficult to suppress. In this case, the liquidfilling the space between the test optical system and the plano-convexmember is water. Water is lower in refractive index than oil. Therefore,the refractive index difference between the liquid and the plano-convexmember cannot satisfactorily be reduced when the liquid is water.

More specifically, when the test optical system is a water-immersionoptical system, the reflection at the plane surface of the plano-convexmember is about 0.4%. Because the reflection at the convex surface ofthe plano-convex member is about 4%, noise light accounts for about 10%of the overall reflected light. Whether or not interference fringesproduced by noise light accounting for about 0.4% of the test lightaffect the measurement depends on the required measurement accuracy.However, such noise light cannot be ignored at least when high-precisionmeasurement is demanded.

Incidentally, interference is a phenomenon that occurs when thedifference in optical path length between the reference light path andthe test light path is not longer than the coherence length of the lightsource. In the second arrangement, light that may interfere with thereference light are mainly the test light and the noise light. The testlight and the noise light differ from each other in that the test lightreturns to the test optical system after traveling between the plane andconvex surfaces of the plano-convex member, whereas the noise lightreturns to the test optical system after being reflected from the planesurface of the plano-convex member. That is, in comparison as to theoptical path length from the light source to the image pickup device,the optical path length of the test light is longer than that of thenoise light by 2×n_(opt)×d.

Therefore, in the second arrangement, the test light path and thereference light path are formed so that distances through which the testlight and the reference light travel to reach the image pickup deviceare approximately equal to each other. Further, a light source having ashorter coherence length L than 2×n_(opt)×d is used. With thisarrangement, because the optical path lengths of the test light and thereference light are approximately equal to each other, the optical pathdifference is substantially zero. In other words, the optical pathdifference is smaller than the coherence length of the light source.Therefore, the test light and the reference light produce interferencefringes. On the other hand, the optical path difference between thenoise light and the reference light is larger than 2×n_(opt)×d. That is,the optical path difference exceeds the coherence length L of the lightsource. Therefore, the noise light and the reference light produce nointerference fringes.

Consequently, with the second arrangement, such an intensitydistribution is formed on the image pickup device that an interferencecomponent (bright-dark component) formed by the test light and thereference light is superimposed on a background component(uniform-brightness component) due to the noise light. It should benoted, however, that the intensity distribution is the summation of theintensities of the two components. Therefore, the background componentcan be removed (subtracted) by image processing executed at a subsequentstep. Accordingly, it is possible to take out only interference fringesformed by the test light and the reference light. Thus, the secondarrangement can remove the noise component due to the noise light and istherefore capable of complying with the requirements of the measurementof a high-precision optical system.

In the case of a Fizeau interferometric optical system such as thatshown in FIG. 8, however, there is an optical path difference betweenthe reference light path and the test light path from the beginning.Therefore, when a Fizeau interferometric optical system is used in thesecond arrangement, it may be impossible to obtain interference fringesand hence impossible to perform the intended measurement. For thisreason, an interferometric optical system wherein the optical pathlengths of the test light path and the reference light path can be madeequal to each other is preferably used in the second arrangement, forexample, a Michelson interferometric optical system or a Mach-Zehnderinterferometric optical system.

It should be noted that if a laser having a long coherence length isused as a light source, for example, it is necessary in order to satisfythe condition (2) to increase the wall thickness d of the plano-convexmember. If the wall thickness d is increased, the plano-convex memberbecomes large in size. Hence, the production of the plano-convex memberbecomes difficult. In addition, the measuring apparatus is caused toincrease in size. Therefore, it is preferable to use a light sourcehaving a short coherence length, that is, a low-coherence light source,e.g. a super-luminescent diode, a light-emitting diode, or a mercurylamp.

In the first and second arrangements, it is also preferable to satisfythe following condition (3):

|n _(med) −n _(opt) |×|r−d|≦0.01 mm  (3)

where: n_(med) denotes the refractive index of a medium lying betweenthe test optical system and the plano-convex optical member; n_(opt)denotes the refractive index of the plano-convex optical member; rdenotes the radius of curvature of the plano-convex optical member; andd denotes the wall thickness of the plano-convex optical member.

The condition (3) is a condition for reducing aberration occurring inthe plano-convex member. The term “aberration occurring in theplano-convex member” as used herein means aberration that occurs whenthe configuration of the plano-convex member deviates from a perfecthemisphere. More specifically, it is aberration occurring owing to thedifference of the wall thickness of the plano-convex member from apredetermined wall thickness.

When aberration occurs in the plano-convex member, the aberrationintroduced into the wavefront of the test light is the sum of aberrationoccurring in the test optical system and the aberration occurring in theplano-convex member. As has been stated above, information concerningthe wavefront of the test light appears as interference fringes throughinterference with the reference light. However, in a state whereinterference fringes are formed, the aberration occurring in the testoptical system and the aberration occurring in the plano-convex membercannot be separated from each other. Therefore, it is impossible todetect only the aberration occurring in the test optical system.

If the condition (3) is satisfied, the aberration occurring in theplano-convex member can be reduced. For example, if |r−d| is large, thearrangement is equivalent to a state where a plane-parallel plate ispresent between the test optical system and the plano-convex member.However, because |n_(med)−n_(opt)| is small, the arrangement isequivalent to a state where no plane-parallel plate is present, as hasbeen stated in connection with the condition (1). Accordingly, it ispossible to minimize the aberration occurring in the plano-convexmember. If |r−d| is small, on the other hand, a plane-parallel plate isnot present. Hence, it is possible to minimize the aberration occurringin the plano-convex member. It should be noted that if |r−d| is small,|n_(med)−n_(opt)| can be made somewhat large. However, if|n_(med)−n_(opt)| is large, noise light occurs at the plane surface ofthe plano-convex member. Therefore, it is preferable not to make|n_(med)−n_(opt)| very large.

Thus, if the condition (3) is satisfied, it is possible to suppress atleast aberration occurring owing to the fact that the plano-convexmember is not a perfect hemisphere although there is unwanted reflectedlight from the plane surface of the plano-convex member. It should benoted that the value of |n_(med)−n_(opt)| may be of the order of 0.2 to0.5 in a case where the wavelength is in the visible range and the testoptical system is a water-immersion optical system, although it dependson the material of the plano-convex member. In a case where thewavelength is in the infrared range and the test optical system is anordinary optical system, when silicon is used as the material of theplano-convex member, the value of |n_(med)−n_(opt)| may be about 4. Theterm “ordinary optical system” as used herein means an optical system inwhich the space between the object and the test optical system is filledwith air.

Further, it is desirable that a reflective coating be provided at leaston the convex surface of the plano-convex member among the opticalmembers disposed in the test light path and the reference light path. Inthe arrangement shown in FIG. 8, the concave member 8 generally usesreflection at a surface of a constituent material, e.g. glass. Thus,measurement is carried out with reflected light that accounts for about4% of the incident light. However, such measurement is extremelyinefficient from the viewpoint of utilization efficiency of lightemitted from the light source. It is conceivable to provide a reflectivecoating on the concave surface of the concave member 8 for the purposeof increasing the efficiency. It is, however, difficult to provide areflective coating because of difficulty in ensuring the requiredcoating surface accuracy and so forth, as has been stated above. In thisregard, if the member that is to be provided with a reflective coatingis a plano-convex member as in the present invention, the accuracy ofthe reflecting surface is determined by the surface accuracy of theconvex surface of the constituent material. Extremely speaking, it isnecessary that the reflective coating be merely present over the surfaceof the convex portion of the plano-convex member. Accordingly, itbecomes possible to effectively utilize light from the light sourcewithout technical difficulty regarding coating. Even if noise lightoccurs at the plane surface of the plano-convex member, it is possibleto perform measurement with a favorable SN ratio because the amount oftest light can be increased by providing a reflective coating on theconvex surface.

It is also preferable to provide a reflective coating on the referencemember, which produces reference light. If the absolute values of theintensities of the reference light and the test light are increased inthis way and the intensities of the two light are made substantiallyequal to each other, the contrast of interference fringes formed by thetest light can be increased relative to the contrast of interferencefringes formed by the noise light. Accordingly, it is possible toperform measurement with an even more favorable SN ratio.

Embodiments of the present invention will be described below in detailwith reference to the accompanying drawings.

(First Embodiment)

A first embodiment of the wavefront measuring apparatus according to thepresent invention will be described. FIG. 1 is a diagram showing thearrangement of a wavefront measuring apparatus according to the firstembodiment. The wavefront measuring apparatus 100 is based on the Fizeauinterferometric optical system as in the case of FIG. 8. Therefore, thesame constituent elements as those shown in FIG. 8 are denoted by thesame reference numerals, and a description thereof is omitted. Thewavefront measuring apparatus 100 according to this embodiment uses aplano-convex lens (plano-convex member) 13 in place of the concavemember 8 used in FIG. 8. The test optical system 12 is an immersionoptical system. Therefore, the space between the test optical system 12and the plano-convex lens 13 is filled with an immersion liquid 14.

Test light emitted from the light source 1 and passing through the testoptical system 12 is reflected by the convex surface of the plano-convexlens 13. The reflected test light passes through the test optical system12 again and is reflected by the beam splitter 4 to reach the CCD camera11. Meanwhile, a part of the light emitted from the light source 1 isreflected by the semitransparent mirror (reference member) 6 to reachthe CCD camera 11 as in the case of the test light. By interferencebetween the test light and the reference light, interference fringes areformed on the CCD camera 11. If the interference fringes are taken intoan image analyzer to perform each analysis, it is possible to measurethe wavefront of the light passing through the test optical system, i.e.various aberrations occurring in the test optical system. Thus, thewavefront measuring apparatus according to this embodiment can performwavefront measurement on an immersion optical system by the sameoperation as in the conventional wavefront measuring apparatus.

In this embodiment, the light source 1 is a He—Ne laser, and theimmersion liquid 14 is oil. The refractive index n_(liq) (n_(med)) ofthe immersion liquid 14 is 1.513 at the wavelength of 633 nm. Therefractive index n_(opt) of the vitreous material of the plano-convexlens 13 is 1.51462 at the wavelength of 633 nm. Accordingly, therefractive index difference |n_(liq)(n_(med))−n_(opt)| between the twoat the wavelength of 633 nm is 0.00162, which satisfies the condition(1) in the present invention. Thus, the wavefront measuring apparatusaccording to this embodiment satisfies the condition (1). Hence, lightreflected at the plane surface 13 a of the plano-convex lens 13 issubstantially zero. Further, because the value of|n_(liq)(n_(med))−n_(opt)| is smalI, the demanded tolerances(manufacturing errors) for the wall thickness of the plano-convex lens13 are not very strict.

Regarding the plano-convex lens 13 used in this embodiment, the radius rof curvature of the convex surface is 12.504±0.002 mm, and the wallthickness d is 12.5 mm. Hence, the absolute value of the difference,i.e. |r−d|, is 0.006 mm at the maximum. Therefore, the condition (3) issatisfied. It should be noted that, as the plano-convex lens 13, it ispreferable to use a plano-convex lens made so that the absolute value ofthe difference between the curvature radius r and the wall thickness d,i.e. |r−d|, is within 0.03 mm.

However, it is a matter of course that the value of |r−d| depends on theoptical performance of the test optical system 12. In a case where thetest optical system 12 is a high-precision and high-performance opticalsystem, for example, the accuracy required for measurement becomesstrict. In such a case, therefore, it is necessary to reduce the valueof |r−d|.

When the test optical system 12 is an optical system using water as animmersion liquid as in the case of a water-immersion objective, theplano-convex lens 13 should preferably be made by using a resin materialsuch as Cytop (trade name; manufactured by Asahi Glass Company, Ltd.).By doing so, measurement similar to the above can be performed. Thereason for this is as follows. At the wavelength of 633 nm, therefractive index of the immersion liquid 14, which is water, is 1.331,and the refractive index of Cytop is 1.34. Hence,|n_(liq)(n_(med))−n_(opt)|=0.009. Thus, the condition (1) is satisfied.

(Second Embodiment)

A second embodiment of the wavefront measuring apparatus according tothe present invention will be described. FIG. 2 is a diagram showing thearrangement of a wavefront measuring apparatus according to the secondembodiment. In the second embodiment, the wavefront measuring apparatus200 is based on the Michelson interferometric optical system. In thecase of the Michelson interferometric optical system, a laser beam fromthe light source 1 is separated into transmitted light propagatingrectilinearly through the beam splitter 4 and reflected lightpropagating in a direction different from the direction of thetransmitted light (in the second embodiment, the reflected lightpropagates in a direction perpendicular to the direction of thetransmitted light).

In this embodiment, the semitransparent mirror 6 is disposed on thetransmitted light side to produce reference light. Accordingly, theoptical path between the beam splitter 4 and the semitransparent mirror6 is a reference light path. On the other hand, the objective 12 and theplano-convex lens 13 are disposed on the reflected light side. The spacebetween the objective 12 and the plano-convex lens 13 is filled with animmersion liquid 14. Accordingly, the optical path between the beamsplitter 4 and the plano-convex lens 13 is a test light path.

In contrast to the first embodiment, the collimator lens 5 in thisembodiment is disposed closer to the light source 1 than the beamsplitter 4. Therefore, light passing through the beam splitter 4 orreflected therefrom is a parallel beam. In addition, a first relay lens10 a is placed between the beam splitter 4 and the spatial filter 9.Light (interference light) passing through the spatial filter 9 iscollected through a second relay lens 10 b to form interference fringeson the CCD camera 11. Thus, interference fringes finally obtained withthe CCD camera 11 are the same as in the first embodiment. Accordingly,the same measurement as in the first embodiment can be performed. Theinterferometric optical system is not necessarily limited to theMichelson interferometric optical system or the Fizeau interferometricoptical system. Other interferometric optical systems are also usable.

(Third Embodiment)

A third embodiment of the wavefront measuring apparatus according to thepresent invention will be described. FIG. 3 is a diagram showing thearrangement of a wavefront measuring apparatus according to the thirdembodiment. In the third embodiment, the wavefront measuring apparatus300 is based on the Michelson interferometric optical system in the sameway as in the second embodiment.

In this embodiment, a low-coherence light source is used as a lightsource 15. Examples of low-coherence light sources usable in the presentinvention include a laser having a very short coherence length incomparison to the laser light source 1 used in the first and secondembodiments, a mercury lamp, and a super-luminescent diode. In thisembodiment, because a low-coherence light source is used, measurement iscarried out in a state where the optical path length from the beamsplitter 4 to the convex surface of the plano-convex lens 13 and theoptical path length from the beam splitter 4 to the semitransparentmirror 6 are adjusted so as to be equal to each other. There are twomethods of adjusting the optical path length: 1) moving thesemitransparent mirror 6 along the optical axis; and 2) moving theobjective 16 and the plano-convex lens 13 together as one unit along theoptical axis. The method in which the semitransparent mirror 6 is movedis easier.

It should be noted that, in the foregoing arrangement, the plano-convexlens 13 is made so that the round-trip optical path length (2×n×d)between the plane surface 13 a and the convex surface 13 b is longerthan the coherence length of the light source 15. In other words, thedifference between the round-trip optical path length from the beamsplitter 4 to the plane surface 13 a of the plano-convex lens 13 and theround-trip optical path length from the beam splitter 4 to thereflecting surface of the semitransparent mirror 6 is longer than thecoherence length of the light source 15. Therefore, light travelingalong these optical paths do not interfere with each other. Accordingly,even if there is reflected light from the plane surface 13 a of theplano-convex lens 13, the reflected light is uniformly present in theform of a background for interference fringes. Therefore, the reflectedlight from the plane surface 13 a has no effect on the interferencefringes as the object of measurement. Further, because the backgroundcomponent can be removed by post-processing, it is possible to improvethe accuracy of fringe analysis made thereafter if appropriateprocessing is executed to remove the background component.

In this embodiment, the coherence length L of the light source 15 is 150μm. The wall thickness d of the plano-convex lens 13 is 12.5 mm, and therefractive index n_(opt) of the plano-convex lens 13 is 1.51825.Therefore, the condition (2) is satisfied.

In this embodiment, the test optical system 16 is a water-immersionoptical system. Therefore, the immersion liquid 17 filling the spacebetween the test optical system 16 and the plano-convex lens 13 iswater. The refractive index n_(liq) (n_(med)) of water for the spectrale-line is 1.334, and the refractive index n_(opt) of the plano-convexlens 13 for the spectral e-line is 1.51825. Hence,|n_(liq)(n_(med))−n_(opt)|=0.18425. Thus, the condition (1) is notsatisfied. However, the curvature radius r of the plano-convex lens 13is 12.504±0.002 mm. Hence, |r−d|=0.006 mm at the maximum. Therefore, thecondition (3) is satisfied.

Thus, in this embodiment, the influence of aberration occurring in theplano-convex lens 13 can be eliminated although it is not possible tosatisfactorily eliminate the influence on interference fringes of thereflected light from the plane surface 13 a of the plano-convex lens 13.It should be noted that, as the plano-convex lens 13, it is preferableto use a plano-convex lens made so that the absolute value of thedifference between the curvature radius r and the wall thickness d, i.e.|r−d|, is within 0.03 mm.

(Fourth Embodiment)

A fourth embodiment of the wavefront measuring apparatus according tothe present invention will be described. FIG. 4 is a diagram showing thearrangement of a wavefront measuring apparatus according to the fourthembodiment. In the fourth embodiment, the wavefront measuring apparatus400 is based on the Michelson interferometric optical system in the sameway as in the third embodiment. This embodiment differs from the thirdembodiment in that the test optical system 18 is not an immersionoptical system but an ordinary optical system. The term “ordinaryoptical system” as used herein means an optical system in which thespace between the test optical system and the sample is filled with air,which is also referred to as “dry optical system”.

Even when the test optical system 18 is such an ordinary optical system,if a light source having a short coherence length is used as the lightsource 15, reflected light from the plane surface 13 a of theplano-convex lens 13 does not contribute to the interference, as hasbeen stated in connection with the third embodiment. That is,interference fringes formed on the CCD camera 11 contain wavefrontinformation concerning only the test optical system. Therefore,wavefront measurement can be performed on an ordinary optical systemwith high accuracy.

In this embodiment, because the space between the test optical system 18and the plano-convex lens 13 is filled with air, the refractive indexn_(opt) is 1.51825, and the refractive index n_(med) is 1. Hence,|n_(liq)(n_(med))−n_(opt)|=0.51825. On the other hand, the wallthickness d of the plano-convex lens 13 is 12.5 mm, and the curvatureradius r of the plano-convex lens 13 is 12.504±0.002 mm. Hence,|r−d|=0.006 mm at the maximum. Therefore, the condition (3) issatisfied.

Thus, in this embodiment, the influence of aberration occurring in theplano-convex lens 13 can be eliminated although it is not possible tosatisfactorily eliminate the influence on interference fringes of thereflected light from the plane surface 13 a of the plano-convex lens 13.It should be noted that, as the plano-convex lens 13, it is preferableto use a plano-convex lens made so that the absolute value of thedifference between the curvature radius r and the wall thickness d, i.e.|r−d|, is within 0.01 mm.

(Fifth Embodiment)

A fifth embodiment of the wavefront measuring apparatus according to thepresent invention will be described. FIG. 5 is a diagram showing thearrangement of a wavefront measuring apparatus according to the fifthembodiment. In the fifth embodiment, the wavefront measuring apparatus500 is based on a modification of the Mach-Zehnder interferometricoptical system. Light emitted from the light source 1 is split intotransmitted light and reflected light through the beam splitter 4. Inthis embodiment, the optical path of the light reflected from the beamsplitter 4 to reach a mirror 20 a is a reference light path. The mirror20 a, together with a mirror 20 b, is disposed in the reference lightpath to lead reference light to a beam combiner 19. On the other hand,the light passing through the beam splitter 4 passes through the beamcombiner 19 to enter the test optical system 12. This is a test lightpath.

In this embodiment, the light source 1 is a laser having a longcoherence length, and the test optical system 12 is an immersion opticalsystem. However, a light source 15 having a short coherence length maybe used as a light source. When the light source 15 having a shortcoherence length is used, it is necessary to adjust the respectiveoptical path lengths of the reference light path and the test lightpath. In this embodiment, optical path length adjustment is performed bymoving the test optical system 12, the immersion liquid 14 and theplano-convex lens 13 together as one unit along the optical axis, asshown by the double-headed arrow.

Sixth Embodiment

A sixth embodiment of the wavefront measuring apparatus according to thepresent invention will be described. FIG. 6 is a diagram showing thearrangement of a wavefront measuring apparatus according to the sixthembodiment. The wavefront measuring apparatus 600 according to the sixthembodiment is characterized in that interferometric optical systemsections (including a reference light path and a test light path) areformed from a prism. By integrating the interferometric optical systemsections into a prism 21, the interferometric optical system can beprevented from being readily affected by the influence of disturbance,such as the fluctuation of air.

(Seventh Embodiment)

A seventh embodiment of the wavefront measuring apparatus according tothe present invention will be described. FIG. 7 is a diagram showing thearrangement of a wavefront measuring apparatus according to the seventhembodiment. The basic structure of the wavefront measuring apparatus 700according to the seventh embodiment is the same as that of the firstembodiment. However, the seventh embodiment differs from the firstembodiment in that the test optical system 12 is a water-immersionoptical system, and a plano-convex lens 22 has aluminum evaporated ontoa convex surface 22 b thereof.

Light emitted from the light source 1 separates into reference lightreflected from the semitransparent mirror 6 and test light passingthrough the semitransparent mirror 6 and further passing through thetest optical system 12. The test light enters the plano-convex lens 22and reaches the convex surface 22 b. Because the convex surface 22 b isprovided with a reflective coating (aluminum vapor deposition). In thisembodiment, the reflectance at the convex surface 22 b is raised toabout 90% by the reflective coating. Meanwhile, the semitransparentmirror 6 is also provided with a reflective coating that provides areflectance of about 35%. Although measurement can be performed on animmersion optical system and so forth even if the reflective coatingsare not provided, it is preferable to provide the reflective coatingswith a view to increasing the measurement accuracy.

The arrangement in which a reflective coating is provided on the convexsurface of the plano-convex lens is also advantageous from the followingviewpoint. Let us consider a case where the concave surface of theconcave member 8 is provided with a reflective coating in theconventional arrangement. In this case, test light is reflected at theair-contact surface side of the reflective coating. Accordingly, it isnecessary to ensure high coating accuracy (i.e. the accuracy andsmoothness of the coating configuration) for the air contact surfaceside of the reflective coating. Therefore, an advanced coating techniqueis required. In contrast, in a case where a reflective coating isprovided on the convex surface of a plano-convex lens as in thisembodiment, test light is reflected at an interface surface between theconvex surface 22 b of the plano-convex lens 22 and the reflectivecoating. In this regard, the surface accuracy (i.e. the accuracy of thesurface configuration and the surface roughness) of the interface isdetermined by the forming accuracy of the convex surface 22 b of theplano-convex lens 22. In other words, the surface accuracy of theinterface does not depend on the surface accuracy of the air contactsurface side of the reflective coating. Accordingly, the reflectivecoating in this embodiment is allowed to be nonuniform in coatingthickness. Extremely speaking, it is only necessary that the reflectivecoating be present over the convex surface. Thus, no advanced coatingtechnique is required, and the amount of test light can be increasedeasily.

It will be apparent that a reflective coating may be provided on theconvex surface 13 b of the plano-convex lens 13 in the first to sixthembodiments. In this case, the semitransparent mirror 6 shouldpreferably be provided with a reflective coating as in the case of theconvex surface 13 b of the plano-convex lens 13. A metal other thanaluminum is also usable for coating. It is also possible to use adielectric multilayer coating.

As has been stated above, the wavefront measuring apparatus according tothe present invention allows wavefront measurement to be performed on animmersion optical system with comparable ease of handling to that of theconventional wavefront measuring apparatus using a concave member.

Further, it is possible to suppress the generation of noise light at aplane surface of a plano-convex member for reflecting test light. It isalso possible to minimize the occurrence of aberration due to an errorin configuration of the plano-convex member. Accordingly, a wavefrontaffected only by aberrations of the test optical system can be detectedin the form of interference fringes.

What I claim is:
 1. A wavefront measuring apparatus for measuring awavefront of light passing through a test optical system, said wavefrontmeasuring apparatus comprising: a light source; a reference light pathin which a reference member for producing reference light is disposed; atest light path in which said test optical system is disposed; and aplano-convex optical member disposed in said test light path in such amanner that a plane surface thereof faces toward said test opticalsystem, said plano-convex optical member having a wall thicknessapproximately equal to a radius of curvature of a convex surfacethereof; wherein a space between said test optical system and saidplano-convex optical member is filled with a liquid.
 2. A wavefrontmeasuring apparatus according to claim 1, wherein the followingcondition (1) is satisfied: |n_(liq)−n_(opt)|≦0.1  (1) where n_(liq)denotes a refractive index of said liquid, and n_(opt) denotes arefractive index of said plano-convex optical member.
 3. A wavefrontmeasuring apparatus according to claim 1, wherein the followingcondition (3) is satisfied: |n _(med) −n _(opt) |×|r−d|≦0.01 mm  (3)where: n_(med) denotes a refractive index of a medium lying between saidtest optical system and said plano-convex optical member; n_(opt)denotes a refractive index of said plano-convex optical member; rdenotes a radius of curvature of said plano-convex optical member; and ddenotes a wall thickness of said plano-convex optical member.
 4. Awavefront measuring apparatus according to claim 1, wherein at least theconvex surface of said plano-convex optical member is provided with areflective coating.
 5. A wavefront measuring apparatus for measuring awavefront of light passing through a test optical system, said wavefrontmeasuring apparatus comprising: a light source; a reference light pathin which a reference member for producing reference light is disposed; atest light path in which said test optical system is disposed; and aplano-convex optical member disposed in said test light path in such amanner that a plane surface thereof faces toward said test opticalsystem, said plano-convex optical member having a wall thicknessapproximately equal to a radius of curvature of a convex surfacethereof; wherein an optical path length of said test light path and thatof said reference light path are approximately equal to each other, andthe following condition (2) is satisfied: L<2×n_(opt)×d  (2) where: Ldenotes a coherence length of said light source; d denotes a wallthickness of said plano-convex optical member; and n_(opt) denotes arefractive index of said plano-convex optical member.
 6. A wavefrontmeasuring method for measuring a wavefront of light passing through atest optical system by using a measuring optical system having a lightsource, a reference light path in which a reference member for producingreference light is disposed, and a test light path in which said testoptical system is disposed, wherein a plano-convex optical member isdisposed in said test light path in such a manner that a plane surfacethereof faces toward said test optical system, said plano-convex opticalmember having a wall thickness approximately equal to a radius ofcurvature of a convex surface thereof, and a space between said testoptical system and said plano-convex optical member is filled with aliquid.
 7. A wavefront measuring method according to claim 6, whereinthe following condition (1) is satisfied: |n _(liq) −n _(opt)|≦0.1  (1)where n_(liq) denotes a refractive index of said liquid, and n_(opt)denotes a refractive index of said plano-convex optical member.
 8. Awavefront measuring method according to claim 6, wherein the followingcondition (3) is satisfied: |n _(med) −n _(opt) |×|r−d|≦0.01 mm  (3)where: n_(med) denotes a refractive index of a medium lying between saidtest optical system and said plano-convex optical member; n_(opt)denotes a refractive index of said plano-convex optical member; rdenotes a radius of curvature of said plano-convex optical member; and ddenotes a wall thickness of said plano-convex optical member.
 9. Awavefront measuring method according to claim 6, wherein at least theconvex surface of said plano-convex optical member is provided with areflective coating.
 10. A wavefront measuring method for measuring awavefront of light passing through a test optical system by using ameasuring optical system having a light source, a reference light pathin which a reference member for producing reference light is disposed,and a test light path in which said test optical system is disposed,wherein a plano-convex optical member is disposed in said test lightpath in such a manner that a plane surface thereof faces toward saidtest optical system, said plano-convex optical member having a wallthickness approximately equal to a radius of curvature of a convexsurface thereof, an optical path length of said test light path and thatof said reference light path are made approximately equal to each other,and the following condition (2) is satisfied: L<2×n_(opt)×d  (2) where:L denotes a coherence length of said light source; d denotes a wallthickness of said plano-convex optical member; and n_(opt) denotes arefractive index of said plano-convex optical member.